Differences in engineers’ brain activity when CAD modelling from isometric and orthographic projections

A way of presenting information in visual representations of technical systems influences the progress and the outcome of the engineering design process. Consequently, improving the means by and through which information is utilised during the process is one suggested approach to advancing engineering design. Engineers’ interaction with visual representations of technical systems is mainly visual and virtual. Although such interactions are cognitively complex, little is known about cognition (mental information processing) underlying the utilisation of design information during engineering design. To narrow the research gap, this study explores how visual representations of technical systems affect engineers’ brain activity while generating computer-aided design (CAD) models based on them. More precisely, the brain activity of 20 engineers is captured and analysed using electroencephalography (EEG) during the visuospatially-intensive design tasks of CAD modelling in two conditions; when technical systems are presented with orthographic and isometric projections in technical drawings. The results imply the sensitivity of engineers’ brain activity in CAD modelling to the visual representation from which a technical system is interpreted. In particular, significant differences are found in theta, alpha, and beta task-related power (TRP) over the cortex when interpreting the technical drawings and CAD modelling from them. Furthermore, the results reveal significant differences in theta and alpha TRP when considering the individual electrodes, the cortical hemispheres, and the cortical areas. In particular, theta TRP over the right hemisphere and the frontal area seems essential in distinguishing neurocognitive responses to the orthographic and isometric projections. As such, the conducted exploratory study sets the foundations for exploring engineers’ brain activity while performing visuospatially-intensive design tasks, whose segments are relatable to the aspects of visuospatial thinking. Future work will explore brain activity in other design activities that are highly visuospatial, with a larger sample size and an EEG device of a higher spatial resolution.

www.nature.com/scientificreports/ RQ1: Is the brain activity of mechanical engineers different when interpreting isometric (condition #1) and orthographic (condition #2) projections in technical drawings? RQ2: Is the brain activity of mechanical engineers different when generating CAD models from isometric (condition #1) and orthographic (
With reference to the results of the previous studies, an increase in theta and beta frequency bands and a decrease in alpha frequency band power (compared to the baseline) may be expected during both CAD task segments and for both conditions (#1 and #2). Additionally, larger theta and beta power increase and smaller alpha decrease may be expected when interpreting the orthographic projection and generating a 3D CAD model from it (compared to the condition with the isometric projection). Furthermore, it is assumed that the right hemisphere (RH) will be more activated than the left one as the RH in right-handed human beings seems specialised for processing visuospatial information 33 . For instance, Roberts and Ann Bell 34 reported a larger alpha power decrease in the right than the left parietal area during visuospatial information processing, thus implying the importance of the RH and the frontal cortical area. Regarding the cortical areas, higher activation in frontal theta (increase), frontal beta (increase), and rear alpha (decrease) may be expected during both CAD task segments when using orthographic projections 26,35 . Such activation is often related to higher mental effort and cognitive workload, which is expected when using orthographic projection. Namely, it is assumed that one must allocate additional cognitive resources to mentally manipulate 2D information presented in the three 2D views of the orthographic projection to combine into a 3D mental model of the represented object 36 .

Methods
Study participants. The study recruited 20 male subjects to participate in the experiment. The inclusion criteria were: being a mechanical engineer, right-handed, and having at least a basic knowledge of using Solid-Works as CAD modelling software. In addition, the participants were instructed to refrain from coffee and caffeine beverages at least two hours before the experiment. Data of the two participants from the original sample (n = 20) were discarded from the data analysis. One participant reported diagnosed neurological issues, and the other left-handedness.
The participants filled in the questionnaire on demographics and prior-experiment experience as part of the experimental procedure. The results showed that participants ranged in age from 25 to 30, with a median (Med) of 27.50 and a median absolute deviation (MAD) of 1.34. Prior professional engineering experience of participants ranged from 0 to 72 months, with Med = 21.50 and MAD = 16.84. All the participants finished the same CAD course (as a part of their engineering studies) in which they learned how to use SolidWorks, a professional computer tool for 3D modelling and engineering documentation. They, on average, spend 10% of their work time on CAD modelling (Med = 10, MAD = 13.16), ranging from 0 to 70%. The distribution of their CAD modelling frequency was as follows: never (10%), rarely (20%), sometimes (25%), often (40%), and always (5%). Experimental tasks. The study incorporated two CAD tasks in which the participants were asked to generate 3D CAD models of two parts based on their technical drawings. The complexity of the CAD tasks was kept the same, as defined by the type and the number of features constituting the resulting 3D models 37 . The parts consisted of the following features: a cuboid, a fillet, a chamfer, a through hole, a slot, and three through slots. The parts are presented in Fig. 1.  www.nature.com/scientificreports/ Participants were not restricted by the number or types of features when generating 3D models. In one CAD task, the technical system (part 1) was presented with the single-view isometric projection in the technical drawing (condition #1). In the other CAD task, the orthographic projection (condition #2) with three main views (front, top, right) in the first angle was used as a 2D visual representation (part 2). Hence, the projection type used to present the parts in technical drawings was an independent variable. In both cases, technical drawings were mediated by the monitor screen as the 2D interface. Experimental setup. The experiment was conducted using one high-performance computer, two 23.8'' monitor screens (resolution of 1920 × 1080 pixels, refresh rate of 60 Hz), and a keyboard and a mouse as the interaction devices. The technical drawings and instructions with detailed explanations of what should be done in each task and step were presented through the PsychoPy 38 application on the left monitor. The CAD modelling was conducted in SolidWorks software, presented on the right monitor. As presented in Fig. 2, both screens were recorded for the entire experiment duration.
Experimental procedure. The experimental procedure consisted of 15 steps, shown in Fig. 3. Firstly, the participants were introduced to the equipment and the experimental procedure. Next, participants were asked to sign a consent. The informed consent was obtained from all the participants. In the third step, the EEG headset was set up. The participants continued to the CAD tasks when the contact and EEG data quality were satisfactory (according to the metrics defined by Emotiv and indicated with green colour within the data gathering application).
The parts and the CAD tasks were the same for all the participants. The CAD tasks were not time limited. Each participant was asked to generate 3D CAD models of three parts in total. All the participants started with an introductory CAD task that served as a warm-up task for familiarisation with the interaction devices and CAD environment. The introductory CAD task consisted of seven actions: generation of three sketches and four features. The resulting 3D model contained a cuboid, a cylinder, a through hole, and chamfers. After the introductory CAD task, participants were instructed to generate 3D CAD models of two parts based on their drawings (as explained in the previous subsection). Each participant had one trial in each condition (orthographic or isometric projection). However, the order of the conditions was controlled; half of the participants (group 1) first generated a 3D CAD model of part 1 (from its isometric projection) and continued to part 2 (from its orthographic projection). The order was reversed for the other half of the participants (group 2). The randomized division was motivated by the goal of bypassing the potential bias of the previous task and cognitive fatigue as its consequence. Each CAD task was preceded and followed by a baseline task. For the baseline task, participants were asked to stare at the fixation cross on the monitor display until it disappeared (after 20 s). The slides with  Data pre-processing. The EEG data processing was conducted in MATLAB using the EEGLAB toolbox 40 .
An original script for data processing was developed according to the pipelines described by Li et al. 41 , Vieira et al. 19 , and Jia et al. 42 . In the first step, DC offset specific for Emotiv EPOC + devices was removed with the infinite impulse response (IIR) filter (0.16 Hz first order high-pass filter). Secondly, frequencies outside the 4-45 Hz range were removed with the finite impulse response (FIR) filter. After that, muscle artefacts were removed with the blind source separation (BSS) technique based on canonical correlation analysis (CCA) 43 . The filtering parameters were set as follows: window length of 2.5 s, window shift of 1.2 s, and four as the number of the least correlated components to be removed. In the next step, the EEG recording was segmented into epochs representing the baselines and the tasks. Furthermore, the CAD tasks were segmented into CAD task segment #1-interpretation of a technical system from the 2D visual representation (isometric or orthographic) and CAD task segment #2-generation of a 3D CAD model. The start of the first CAD task segment was defined by the timing of the transition to the slide with 2D representation, and it was derived from the PsychoPy log files. The end of the first CAD task segment corresponds to the start of the second CAD task segment. Namely, the start of the CAD task segment #2 was defined as a moment when the participant started generating the first sketch element (e.g. by drawing the first line) in SolidWorks. An example of segmentation is presented in Fig. 4. The segmentation was followed by removing of the windows (length of 1 s, shift of 1/128 s) with an amplitude exceeding ± 100 µV and/or the threshold value calculated for each participant individually. The threshold was calculated as a value three standard deviations greater than the mean (M) of the entire epoch across the electrodes. In this way, any 1 s long epoch of the EEG data with the M amplitude higher than the calculated threshold (or 100 µV if the absolute threshold value was above it) was discarded. The percentage of the bad EEG data ranged from 0.19 to 4.02% for the condition #1 (Med = 0.83, MAD = 0.76) and from 0 to 9.92% for the condition #2 (Med = 0.76, MAD = 0.94). In the next step, EEG data was divided into theta (4-7 Hz), alpha (8)(9)(10)(11)(12), and beta (13-30 Hz) sub-frequency bands using the FIR filter. After the threshold was applied, the power of EEG signals (Pow) was calculated as the M of the squared values, resulting from the band-pass filtering of the EEG signal and using Fast-Fourier Transformation. The EEGLAB function pop_eegfiltnew, hardcoded to Hamming window, was used for the filtering. In the final pre-processing step, task-related power (TRP) was calculated by subtracting the transformed Pow average of a subject j at an electrode i during a baseline task recorded before each CAD task from the transformed Pow average of a subject j at an electrode i during a CAD task. Hence, TRP values were calculated according to the following expression: Positive TRP values reflect an increase of power during the CAD task (compared to the baseline task), whereas negative TRP values reflect a power decrease 44 .
(1) www.nature.com/scientificreports/ Data analysis. Data analysis was conducted using the R language. Descriptive statistics encompassed the calculation of the Med as a measure of central tendency and MAD as a measure of variability. These parameters were used for data distribution since they are robust to the effects of eventual outliers that potentially persisted after the data pre-processing. Besides, they are more suitable for describing the non-normal distributions (as tested by the Shapiro-Wilk test; p < 0.05). In addition, inferential tests enabled the calculation of differences in duration and TRP values between two CAD modelling task segments (interpreting a technical drawing and generating a 3D CAD model). The analysis encompassed a comparison of the tasks (their segments) based on TRP in three frequency bands (theta, alpha, and beta). A nonparametric repeated measures ANOVA approach was adopted 45,46 to study the differences in TRP values between the projections in each CAD task segment and in each frequency band. Such an approach is based on the Aligned Rank Transform (ART) procedure devised to handle data that violates ANOVA assumptions without inflating the Type I error rates 46 . For each setting (i.e. segment and frequency band), the factors of interest included the projection, electrode, hemisphere (LH and RH), and cortical area (frontal area -FA-and rear area -RA-). The odd-numbered electrodes were grouped under the LH, while the even-numbered ones were under the RH. To compare the TRP values between the cortical areas, the electrodes were distributed as follows: FA: AF3, F7, F3, F4, F8, AF4, FC5, FC6, and RA: O1, O2, P7, P8, T7, T8. The EEG device used in the experiment has good coverage of the FA, but a low spatial resolution in other areas (central, occipital, parietal, and temporal) since only two electrodes are in each of them. Consequently, the division into smaller cortical areas would offer results with a low statistical rigour.
Significant interaction and main effects detected using the nonparametric repeated measures ANOVA were further decomposed into simple interactions, simple main effects and pairwise comparisons to enable further insights. Herein, the pairwise Wilcoxon signed-rank test with Bonferroni correction was used for the posthoc comparisons. In addition to the (adjusted) p-values, the effect size (reported as r-value) of the Wilcoxon singedrank test was calculated by dividing the test statistic by the square root of the number of observations. The p-values and the related effect size are, in the following section, coupled with the test statistic values; partial eta squared for the nonparametric factorial ANOVA test and V for the Wilcoxon signed-rank test. Significant differences are presented graphically in the box plots and numerically in the tables.

Results
The first subsection presents the differences in duration when considering the entire CAD task and its two segments (CAD task segments #1 and #2). The second subsection compares the theta, alpha, and beta frequency band TRP values between conditions #1 and #2 for each CAD task segment. First, the significant differences in TRP values and/or large effect sizes are presented for the CAD task segment #1 in the following order. The significant main effects of projection (i.e. differences among projections considering the entire cortex) are discussed first. Then, the significant main effects and interactions concerning the cortical hemispheres are reported. Finally, the main effects and interactions concerning the cortical areas are listed. The same reporting structure is then www.nature.com/scientificreports/ followed for the CAD task segment #2. Note that, of the explored interactions, only several two-way interactions were found significant and are discussed accordingly (whereas insignificant results are omitted).
Duration. The average duration of the CAD task performance (expressed in seconds) was higher when using the orthographic (Med = 991.04, MAD = 405.29) than the isometric projection (Med = 886.44, MAD = 360.26), as shown in Fig. 5a. However, the difference between the two conditions was not statistically significant. The completion time of the first CAD task segment (interpreting the 2D visual representations) was significantly longer when interpreting the orthographic (Med = 71.5, MAD = 37.81) than the isometric (Med = 32, MAD = 12.6) projection, with V = 170, p = 1.53·10 -5 , and r = 0.87.
Participants spent similar time generating CAD models from the orthographic (Med = 904, MAD = 473.69) and the isometric (Med = 861, MAD = 360.27) projections. Consequently, the completion time of the second task segment was slightly, but not significantly different.
Theta, alpha, and beta TRP. CAD task segment 1: interpreting the projections. Cortex (considering all the 14 electrodes). The nonparametric repeated measures ANOVA revealed a significant main effect of the projection on the TRPs in all three frequency bands. For completeness, the differences in theta, alpha, and beta TRPs over the cortex when interpreting the orthographic and isometric projection were also assessed using the Wilcoxon signed rank test. Figure 6 and Table 1 give further details on these differences.
Cortical hemispheres. The main effect of the cortical hemisphere was found on the theta and alpha TRPs (consult Table 2 for details).
No significant interaction effects between hemisphere and the remaining factors were found in the three frequency bands. Nevertheless, pairwise Wilcoxon signed rank tests with Bonferroni correction revealed a significant difference in alpha TRP between the projections over the RH. In addition, alpha TRP significantly differed between the hemispheres, but only for the condition with the isometric projection, whereas significant differences in theta TRP were found between hemispheres when interpreting both projections ( Fig. 7 and Table 3).  www.nature.com/scientificreports/   www.nature.com/scientificreports/ Cortical areas. No main effects of the cortical area were found in the TRP values of theta, alpha, and beta when interpreting the projections. However, there was a statistically significant interaction between the projection and the cortical area in the theta frequency band (F = 7.56, p = 6.22·10 -3 , and ηp 2 = 1.62·10 -2 ). As shown in Fig. 8a, further analysis of the simple main effects of projection showed significant differences between the theta TRP over the FA, with V = 3603, p = 1·10 -3 , and r = 0.13.

CAD task segment 2: generating the CAD models. Entire cortex (considering all the 14 electrodes).
The main effect of the projection on the TRPs over the cortex was found in all three frequency bands when generating the CAD models. As presented in Fig. 9 and Table 4, theta, alpha, and beta TRPs over the entire cortex significantly different when interpreting the orthographic and isometric projections.   www.nature.com/scientificreports/ Cortical hemispheres. The main effect of the cortical hemisphere was found on the TRPs in all three frequency bands (see Table 5). Furthermore, the interaction effect of the projection and cortical hemisphere was found in theta (F = 5.14, p = 2.38×10 -2 , and ηp 2 = 1.11·10 -2 ) and alpha (F = 3.73, p = 0.05, and ηp 2 = 8.05·10 -3 ) frequency bands. Decomposing the two-way interaction into simple main effects revealed the significant differences between the projections in the theta TRPs over both hemispheres. In other words, the simple main effect of the projection was significant both in the LH (V = 2492, p = 2.41·10 -4 , r = 0.33) and the RH (V = 1414, p = 3.05·10 -10 , r = 0.56), as presented in Fig. 10a. In addition, simple main effect of the cortical hemisphere was significant in the condition with the isometric projection (V = 6511, p = 9.88·10 -2 , and r = 0.55).
At the electrode level, difference between the LH and the RH was found in all electrodes for the isometric projection condition. These electrodes are presented in Table 6.
In addition, the interaction effect of the projection and cortical hemisphere area was found for alpha frequency band. Decomposing this interaction into simple main effect of the projection revealed the differences in alpha TRP over the RH, as presented in Fig. 10b, with V = 2964, p = 1.2·10 -2 , r = 0.23. Furthermore, the simple main effect of the cortical hemisphere was found on alpha TRP for the condition with the isometric projection, with   www.nature.com/scientificreports/ V = 5713, p = 3.07·10 -5 , r = 0.37. Finally, significant differences in alpha TRP between the LH and the RH were found at several electrodes presented in Table 7.
Cortical area. The main effect of the cortical area was found on theta TRP, with F = 13.88, p = 2.19·10 -4 , and ηp 2 = 2.94·10 -2 . Furthermore, the interaction effect of the projection and cortical area was found in the same (theta) frequency band (F = 9.99, p = 1.67·10 -3 , and ηp 2 = 2.13·10 -2 ). Further decomposing the interaction into the simple main effects revealed significant differences in theta TRP over both the FA and the RA when compared between the projections (see Fig. 11 and Table 8). In addition, the simple main effect of the cortical area was found on theta TRP when generating the CAD model from the orthographic projection.
Individual electrodes. For completeness, the interaction among projection and individual electrodes (encoded as: AF3, AF4, F3, F4, F7, F8, FC5, FC6, P7, P8, T7, T8, O1 and O2) was studied. No significant interaction between projection and individual electrodes factors was found in any of the bands. Nevertheless, pairwise Wilcoxon signed rank tests with Bonferroni correction identified several electrodes at which theta TRP differs significantly between the projections (Fig. 12 and Table 9).  Figure 11. (a) Theta TRP, (b) Alpha TRP, and (c) Beta TRP over the cortical areas when generating the 3D CAD models. www.nature.com/scientificreports/

Discussion
Engineers' brain activity was captured and analysed during the visuospatially-intensive design tasks of CAD modelling in two conditions-when technical systems were presented with isometric (condition #1) and orthographic (condition #2) projections in technical drawings. An increase in theta and beta TRP (compared to the baseline) was expected in both CAD task segments (interpreting the projections in technical drawings as segment #1 and generating the 3D CAD models from them as segment #2) since it has often been related to cognitive processing of visuospatial information [22][23][24][25] . In line with that assumption, beta TRP increased in both CAD task segments. However, theta TRP increased when generating the 3D CAD models while surprisingly decreased when interpreting the technical drawings (for both conditions). In addition, alpha TRP decreased only when interpreting the isometric projection. A decrease of alpha TRP was expected in both CAD task segments and for both conditions since such a response has often been observed when processing visuospatial information 27,28 . Moreover, it is generally considered that the RH is specialised for processing visuospatial information in right-handed human beings 33 . Since the tasks used in the experiment are visuospatially-intensive and all the participants included in the analysis were right-handed, higher activation over the RH was expected when performing the CAD tasks. However, the TRP values over the RH were either similar to those of the LH (theta TRP during the first CAD task segment) or lower (in all the other cases). A possible explanation for unexpected brain activity may be related to cognitive characteristics of the CAD tasks and visuospatial information processing in the CAD context. In addition to that argument, Willis et al. 47 suggested that brain activity in visuospatial information processing highly depends on the task requirements posed to the participants (what they should do with presented information) and not only the type of information that should be processed. For example, several studies have reported increased alpha TRP while solving standardized tests related to aspects of visuospatial thinking 23,25,33 . Hence, it may be that increase in alpha TRP revealed when performing CAD tasks is due to using similar cognitive mechanisms. Similarly, revealed brain activity in individual hemispheres aligns with the results reported by Ornstein et al. 33 and Roberts and Ann Bell 34 , who argued that higher activation of the LH may be caused by the analytic strategies specific for the mental rotation tasks. Both mental rotation and interpretation of the visual representation ask for a visual transformation as an aspect of visuospatial thinking (according to Shah et al. 29 ). Hence, it may be that interpretation of visual representations causes higher activation of the LH because of the underlying visual transformation required to conduct it. Furthermore, the dimensionality of visual representation used in the mental rotation task seems to affect alpha TRP in the hemispheres. Namely, the 2D mental rotation task was previously associated with the higher parietal activation in the LH than the RH. On the contrary, the activation was higher in the right than the left parietal area during the 3D mental rotation task 34 . It is yet to be explored why the results diverge across the studies that used the same standardized tests and what it means for the CAD context.
The study hypothesised differences in mechanical engineers' brain activity when interpreting isometric and orthographic projections in technical drawings as well as when generating CAD models from them. EEG results  www.nature.com/scientificreports/ revealed statistical significance and considerable effect sizes in both CAD task segments when analysed across the cortex, cortical hemispheres, cortical areas, and individual electrodes. Thus, both research questions are answered positively, and the hypothesis is confirmed; the brain activity of mechanical engineers was different when interpreting isometric and orthographic projections in technical drawings, as well as when generating 3D CAD models from them. Moreover, the analysis revealed a significant difference in the duration of the first CAD task segment (interpretation of the technical drawing) between the orthographic and isometric projection conditions. In contrast, the time spent generating the 3D geometry with the CAD tool (as the second CAD task segment) was similar in both conditions. Hence, the duration differences imply a significantly faster visual transformation and synthesis 29 (prevalent in the first CAD task segment) from the isometric than the orthographic projection. Considering these differences in the duration of performing the CAD task segments, a more evident effect of the projections on engineers' brain activity was expected when comparing brain activity in CAD task segment #1.
However, the following subsections imply different conclusions. In particular, it seems that the effect of the projections on engineers' brain activity is more evident when generating CAD models than interpreting the technical drawings. Such results may be related to different aspects underlying the two CAD task segments (visual transformation and synthesis for segment #1; visual expression for segment #2). Differences in brain activity between the CAD task segments will be analysed in future work.
Differences in mechanical engineers' brain activity when interpreting the projections. The analysis of the TRP values in all three frequency bands suggested statistically significant differences in brain activity when interpreting the orthographic and isometric projections. In particular, a decrease of theta TRP was smaller when interpreting the orthographic projection (as compared to the baseline). Furthermore, alpha TRP increased when interpreting the orthographic projection while decreased when using the isometric one. Finally, an increase in beta TRP was larger for the orthographic than the isometric projection. The smaller decrease or larger increase in theta and beta TRP confirm the assumption of using more cognitive resources to interpret the technical drawing in which the technical system is presented with the orthographic projection (due to the need to combine three 2D views to a mental 3D model). Such results are in line with previous studies (e.g. the work of Fajen and Philips 36 ). Alpha, on the other hand, behaved differently from what was expected-there was more alpha TRP in condition #2 although it was expected to see alpha TRP decrease with increasing processing demands (imposed by the orthographic projection) 27,28 .
Moreover, mechanical engineers used the cortical hemispheres similarly when interpreting both projections since no significant differences in the TRP values were found between the hemispheres when considering the projections. However, alpha TRP suggests different activation of the RH when interpreting the isometric than the orthographic projection. In addition, a significant difference in alpha TRP was found between the LH and the RH when interpreting the projections. Similarly, significant differences were found in theta frequency band when TRPs were compared between the hemispheres. These results imply the asymmetric hemisphere activation when interpreting the projections.
Differences in mechanical engineers' brain activity when generating the 3D CAD models from the projections. Similarly to the first CAD task segment, the TRP values were significantly different when generating the CAD models from the orthographic and isometric projections in all three frequency bands. However, the difference in TRP values for the CAD task segment #2 between the conditions was reflected in a larger increase when using the orthographic than the isometric projection in all three frequency bands.
Furthermore, significant differences in theta TRP were found over the individual electrodes when comparing the conditions (see Fig. 12). These electrodes were mainly located in the FA, thus implying the important role of theta over the FA in distinguishing the effects of the projections on engineers' brain activity.
The effect of the projections when generating the CAD models was reflected in the theta and alpha TRPs, which both significantly differed for conditions #1 and #2 over the RH. The difference among the projections in theta TRP was also significant over the LH. When comparing theta TRP values over the hemispheres, it is noticeable that a decrease (with respect to the baseline task) is present over the RH and only for the isometric condition. In addition, the differences in TRP values over the LH and the RH were significant in the theta and alpha frequency band when generating the CAD models from the isometric projection. For this condition (#1), the revealed difference in theta TRP between the hemispheres was corroborated by the significant differences among all the seven pairs of the electrodes (as shown in Table 6). At the level of the electrodes, the effect of the hemispheres on the alpha TRP was noticed mainly over those located in the FA (see Table 7).
Considering the cortical areas, significant differences between the conditions were found in theta TRP over both the RA and the FA. Theta TRP over the FA increases in value for the orthographic and a decreased for the isometric projection. Such results are aligned with previous studies on the visual processing of information. For example, Liu et al. 26 reported high theta TRP over the FA during a mental rotations task. An increase in theta TRP over the FA for condition #2 might be related to higher requirements on attention and the level of the cognitive load imposed by interpreting and generating CAD models from the orthographic projection 28 . Higher alpha TRP in the RA for condition #2 in both CAD task segments suggests similar explanations (although differences were not statistically significant). For instance, Gerlic and Jausovec 35 related higher FA alpha power with more efficient processing of presented information and higher alpha activation in the RA (temporal area in particular) with higher cognitive load.
Limitations. Several limitations of the presented study should be noted. The first limitation concerns the sample size. Namely, the statistical analysis was conducted on data from the 18 participants and offered some www.nature.com/scientificreports/ statistical evidence. However, the study should be repeated with more participants to corroborate the recognised differences statistically. In addition, both participants whose data were discarded from the analysis belonged to group two (as defined in Fig. 3), which started CAD modelling from the isometric and then moved to the orthographic projection. A difference in the number of participants within the groups could have affected the lack of statistical significance. Furthermore, the study should be more extensive with the capabilities of the used EEG device. Namely, the EEG device used in the study contained 14 electrodes. Because of the relatively low spatial resolution, the study did not try to relate brain activity and functions of brain areas where the electrodes were located since such analysis may not be reliable 48 .

Conclusion and further work
The presented study showed that engineers' brain activity in interpreting the 2D visual representations of technical systems (technical drawings with orthographic and isometric projections) and CAD modelling from them can be recorded, described, and compared using EEG. The results imply the sensitivity of engineers' brain activity in CAD modelling to the visual representation from which a technical system is interpreted. In particular, when interpreting the technical drawings and CAD modelling from them, significant differences were found in theta, alpha, and beta TRP over the cortex (considering all 14 electrodes cumulatively). Furthermore, the results revealed significant differences in theta and alpha TRP when considering the individual electrodes, the cortical hemispheres, and the cortical areas. In particular, theta TRP over the RH and the FA seems to be essential in distinguishing neurocognitive responses to the orthographic and isometric projections. In further analysis, we will aim to relate brain activity with CAD actions, shorter task epochs, and outcomes. The conducted exploratory study sets the foundations for exploring engineers' brain activity while solving visuospatially-intensive design tasks, whose segments are relatable to the aspects of visuospatial thinking. Future work will explore brain activity in other design activities that are highly visuospatial, with a larger sample size and the EEG device of a higher spatial resolution. As a result, typical brain behaviour(s) for visuospatially-intensive design activities may be understood, described, and used as input when further developing visual representations of technical systems, CAD software, and HCI tools.

Data availability
The datasets generated and analysed during the current study are available in the Zenodo repository at the following link https:// doi. org/ 10. 5281/ zenodo. 71851 67.